Electromagnetic wave attenuator



1954 v. R. LEARNED ELECTROMAGNETIC WAVE ATTENUATOR Filed Sept. 29. 1949 II' ZO INVENTOR VINCENT A. ZtWR/VED Patented Feb. 23, 1954 UNITED STATES ATEN T OFFICE? Vincent R. Learned, Garden .City N. Yj.,, assignor to The Sperry Corporation, a corporationof Delaware Application September 29, 1949, Serial No. 118,604

3 Claims. (Cl. 333-81) This invention relates to adjustable. attenuator apparatus and particularly. to adjustableattenuatorsfor use in coaxial transmission lines.

Conventional adjustable attenuators. for coaxial transmission lines generally employ rounded cards of resistive material inserted into a slot in the coaxial line. However, such attenuators serve to. produce. a very limited amount of attenuation since the resistive. material can beinserted only a limited distance due. to the presence of the inner conductor of the coaxial line.. Also, the insertion loss inherent in. such attenuators is large, thereby introducing an undesired fixed loss in the transmission line.

Copending. application Ser. No. 672,751, filed May 28', 1946, issued as Pat. No. 2,515,228, July 18, 1950, by Joseph V. I-Iupcey discloses variable attenuator apparatus for, coaxial lines which providescomparatively. large attenuation. The I-Iupcey device employs a resistive card which is inserted through a slot in the outer conductor of the coaxial line and either alongside the inner conductor or through a slot in the inner conductor. The Hupcey device has an appreciable insertion loss and is rather expensive to manufacture since the slots and the card-inserting mechanism must be constructed within narrow tolerances in order to obtain accurately determined attenuation.

Accordingly, it is an object of this invention to, provide a simple. adjustable attenuator apparatus. which has a. wide range. of attenuation and. a low insertion loss.

Another object is to provide attenuatorapparatus-for high frequency energy in coaxial lines in which the reflected energy is minimized, while large amounts of attenuation are provided.

Aiurtherobject is to provide attenuator appa ratus-for. high frequency energy in coaxial lines that produces .very small variations in the energy refiectedwith changes in the attenuation or-in the frequency or the energy.

Other objects and advantages of the invention will be apparent from the following description, the appended claims and the drawingain which:

Fig. 1 is an oblique view of the attenuator apparatus;

Fig.2 is a longitudinal sectional view, partially broken away,.of the, apparatus shown in Fig. 1;

Fig. .3 is a sectional view of the apparatus taken along line 3-3 of Fig; 2, and

Fig. 4 is a partial sectionalview oi the apparatus' showing the use of a center conductor of elliptical cross-section.-

Copencling application Ser. No. 84,457; filed 2, March 30, 1.9.49, issuedas Pat. No., 2.5.3.4;437. on December 19,. 1950, by EdwardL. .Ginzton,.dis.- closes and claims the parallel plate highircquency transmission line section employed in, the attenuator of. the presentinvention.

As. disclosed; in the. Ginzton application... by. the conformal transformation Z== R tan g from. a Z-plane to a W-plane, the electric and magnetic field patterns in anordinary coaxial line can be converted to the electric and mag netic field patterns between a conductor of elliptical cross-section and two. infinite planes equidistant irom and parallel to the major axis of the elliptical conductor. R0 is the inner; radius ofthe outer conductor of the. coaxial line inpthe Zeplane, Z represents the radius vector in the Z-plane, W represents the radius vector in the W-plane, and D is the distance between the two infinite planes.

If the characteristic impedance of the com.- bination of the elliptical. conductor and the infinite planes is to have the same characteristic impedance asthe coaxial line,

where R1- is theradius vector of the innerconductor ofthe coaxial line in the Z-plane.

It'has been found that a conductor of circular cross-section can be substituted for the conductor of elliptical cross-section without materially affecting the electric and magnetic fields between the conductor and the infinite planes. The conductor of circular cross-section has a diameterslightly larger than the calculated diameter of the minor axis of a conductor of'elliptical crosssection, and it is determined from the. relation:

R are tan Thus, the combination of two planes and a cylindrical conductor positioned between and spaced equidistant from the two planes may be inserted in a coaxial line without affecting the characteristic impedance of the line or the speed of propagation of electromagnetic energy along the line.

This affords a convenient and practical means for inserting an attenuator element in the transmission line since very little energy is radiated and consequently the insertion loss is small.

The transitions from the coaxial line sections to the flat plate transmission line comprising the two planes and the cylindrical conductor cause fringing electric and magnetic fields at each end of the flat plate line, since the field configuretions are different, and this causes a small amount of energy to be reflected along the line. Gradual tapering of one line into the other is a possible method of transforming one line into the other so as to eliminate reflections. However, this is mechanically awkward, and increases the unsupported length of the inner conductor.

A more satisfactory junction is obtained by joining the outer conductors of the two coaxial lines directly to end plates connecting the parallel planes and placing a circumferential groove in the inner conductor of each coaxial line at the junction between each inner conductor of the coaxial lines and the cylindrical conductor of the flat plate line. The purpose of these grooves is to introduce a small inductive discontinuity at each junction, and these inductive discontinuities serve to balance out the reflections caused by the changes in field configuration at each end of the fiat plate line.

By suitable choice of the dimensions of the grooves, the characteristic impedance of the combined effect of each groove and the discontinuity associated therewith can be made equal to the characteristic impedance of the coaxial line sections and the efiect of the discontinuities substantially eliminated.

The proper dimensions for the groove are determined experimentally. The depth is not critical and is a function of the diameter of the cylindrical conductor of the flat plate line.

Referring now to the drawings, two spaced plates Iii and i2 having adjacent surfaces which are plane and parallel are connected at each end by end plates M and 6 to enclose a rectangular space l8 having a length L sufiicient to accommodate suitable attenuator cards of resistive material.

In order to increase the mechanical strength of the apparatus, side plates In and I2 are preferably joined at the bottom by a base plate which may be an integral part of plates I6 and I2, if desired.

A cylindrical conductor 22 extends the length of space I8 and is spaced midway between plates NJ and [2. If desired, a conductor of elliptical cross-section may be employed as indicated by the conductor 22' of Fig. 4. The ends of conductor 22 are passed through circular holes 24 and is in end plates i l and It, respectively and connected through undercut sections 28 and 30 to the inner conductors of connectors 32 and 34. The outer conductors of connectors 32 and 34 are attached to end plates i l and it, respectively. Each of the connectors 32 and 34 is provided with a dielectric member 36 to support its inner conductor.

Coaxial lines may be coupled to connectors 32 and 34 so as to connect the attenuator apparatus between a suitable source 38 of high frequency energy and a suitable load or utilization device 40.

One or more cards such as a pair of cards 42 and 44 of resistive material are rigidly secured to a longitudinal support 45 which fits snugly into the rectangular space l8. Support 46 may be moved up and down by means of a shaft 48 having a handle 49 attached to its upper end. A bracket 50 is rigidly secured to the top edges of plates [0 and I2 and is provided with a bushing 52 for guiding the movement of the shaft 48. A pointer 54 is rigidly secured to shaft 48 and in conjunction with a scale 56 on bracket 50 it serves to indicate the transverse position of the attenuator cards in space l8.

Resistive cards 42 and 44 are cards of insulating material having a coating of resistive material on one surface of each card. The surfaces containing the resistive material face outward so as to be adjacent to side plates l0 and 32. The inner surfaces which are composed of insulating material are spaced apart a distance equal to the diameter of the conductor 22 so that the cards 42 and 44 are in slidable contact with the inner conductor 22, thereby causing cards 52 and M to be maintained substantially parallel to one another and to plates Ill and i2 and permitting an accurate determination of the attenuation introduced.

The cards 42 and 44 are preferably rounded so as to minimize the energy reflected thereby. In a preferred embodiment which is disclosed in Fig. 2 the edges of the cards are formed as arcs of a circle.

Suitable dimensions for the embodiment of the invention disclosed herein for use in a coaxial line having an impedance of 53.5 ohms are as follows:

D=.l81 inch H=l.625 inches L=2.817 inches N=.500 inch R2=.4-.50 inch S=.012 inch T=.080 inch Cards 42 and 44 are ohms per square.

It will be apparent that various modifications may be made in the apparatus disclosed herein. For example, cards of resistive material of different types may be employed and the cards may be spaced from the inner conductor 22 if desired. The insertion loss of the attenuator may be reduced to a negligible amount by providing means for removing the resistive cards 42 and 44 from the electromagnetic fields in the immediate vicinity of conductor 22. This may be accomplished by enlarging the distance between conductor 22 and the top of plates l0 and i2 and providing a longer shaft 48 and a longer slot to accommodate the point 54, for example.

Since many changes could be made in the above construction and many apparently widely different embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. An attenuator for insertion between two coaxial line sections comprising a metallic member defining a pair of spaced parallel inner surfaces, a pair of end plates closing oif opposite ends of said metallic member, the coaxial line sections having their outer conductors terminating at said end plates, an inner conducting member extending between and connecting the inner conductors of said coaxial line sections, said inner conducting member extending longitudinally in the space between said parallel inner surfaces, a longitudinal support member adjustably positioned between the parallel surfaces parallel to said inner conducting member, a pair of spaced parallel cards of resistive material secured to the support member and positioned on either side of the inner conducting member, and means carried by the metallic member for adjustably positioning the support member.

2. An attenuator as defined in claim 1 wherein said support member is made of conductive material and slidably engages said parallel inner surfaces and said end plates, whereby the support member is secured against lateral movement, and the interior space is substantially entirely bounded by conductive surfaces.

3. In combination, two coaxial line sections having inner and outer conductors, a metallic channel-shaped member defining a pair of spaced parallel inner surfaces, a pair of end plates secured to and closing off the ends of the channelshaped. member, the end plates having holes therethrough opening into the space between the parallel surfaces of the channel-shaped member, one of said coaxial line sections being secured to each of the end plates, the inner conductors of said coaxia1 line sections being aligned with said holes, a conductive rod extending longitudinally between said parallel surfaces and connecting the inner conductors of said coaxial line sections, the diameter of said rod and spacing between the parallel surfaces of the channelshaped member being so proportioned as to provide the same characteristic impedance as the coaxial line sections, a support member adjustably positioned between the parallel surfaces of the channel-shaped member in parallel relation to the rod, a pair of cards of insulating material having their longitudinal edges secured to the support member and extending the length of the channel-shaped member, the cards being positioned on either side of the rod in parallel relationship to said inner surfaces of the channel-shaped member, the adjacent inner surfaces of said cards being in sliding contact with said conductive rod whereby the rod maintains the cards parallel, the opposite outer surfaces of said cards being coated with resistive material, and mounting means between the support and channel-shaped members for adjustably positioning the support member to provide movement toward or away from the rod.

VINCENT R. LEARNED.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,996,186 Afi'el Apr. 2, 1935 2,436,427 Ginzton Feb. 24, 1948 2,454,530 Tiley Nov. 23, 1948 2,491,644 Carlson Dec. 20, 1949 2,515,228 Hupcey July 18, 1950 2,531,777 Marshall Nov. 28, 1950 2,534,437 Ginzton Dec. 19, 1950 FOREIGN PATENTS Number Country Date 606,891 Great Britain Aug. 23, 1948 OTHER REFERENCES Wholey et al.: A New Type of Slotted Line Section, Proceedings of the National Electronics Conference, November 4-6, 1948. Published February 28, 1949, p. 221 relied on. Copy in Div. 48.

Hewlett Packard Co., Electronics, J anuary, 1949, p. 35 relied on. 

